1. Field of the Invention
This invention relates to a testing apparatus to test a Blow Out Preventer (BOP) stack or assembly and to a method of testing using such an apparatus.
2. Description of the Related Art
A BOP assembly is a multi closure safety device which is connected to the top of a drilled and often partially cased hole. The accessible top end of the casing is terminated using a easing spool or wellhead housing upon which the BOP assembly is connected and sealed.
The wellhead and BOP stack (the section in which rams are provided) must be able to contain fluids at a pressure rating in excess of any formation pressures that are anticipated when drilling or when having to pump into the well to suppress or circulate an uncontrolled pressurized influx of formation fluid. This influx of formation fluid is known as a ‘kick’ and restabilizing control of the well by pumping to suppress the influx or to circulate the influx out under pressure is known as ‘killing the well. An uncontrolled escape of fluid, whether liquid or gas, to the environment is termed a ‘blow out’. A blow out can result in a major leak to the environment which can ignite or explode, jeopardizing personnel and equipment in the vicinity, and pollution.
Although normal drilling practices provide a liquid hydrostatic pressure barrier to a kick, a final second mechanical safety barrier is provided by the BOP assembly. The BOP assembly must close and seal on tubular equipment hung or operated through the BOP assembly and ultimately must be capable of shearing and sealing off the well. Wells are typically drilled using a tapered drill string having successively larger diameter tubulars at the lower end. When running a completion or carrying out a workover various diameter of tubulars, coiled tubing, cable and wireline and an assortment of tools are run.
The consequences of any failure of the BOP assembly multi closure barriers and valves, shear and seal devices to correctly operate in an emergency can be far reaching. It is essential to initially contain the kick to prevent a blow out and then be capable of killing the well, and re-establishing control.
To verify the functions and performance of a BOP assembly, stringent tests have to be performed on a regular bases, either daily, weekly or at certain stages of the drilling operation to ensure the BOP is in full working order. When drilling or carrying out well intervention on a subsea well where the wellhead is at the seabed, the subsea BOP attached to the subsea wellhead is connected to a buoyant floating drilling vessel by a riser. A floating drilling vessel should maintain its station vertically above the well to enable well operations to be performed.
Failure to do so caused by weather conditions, current forces, equipment malfunctions, drift off or drive off, fire or explosion, collision or other marine incidents means it is necessary if possible to make the well safe, isolate the well at the seabed and disconnect the riser system. In a severe emergency, shearing any tubulars or equipment in the BOP bore, sealing the well to full working pressure and disconnecting the riser system is required to be achieved in under 30 seconds.
A conventional BOP assembly, surface or subsea, is attached to a wellhead and is provided with a number of ram BOPs to either seal around different set tubular diameters or to shear and seal the bore. These ram BOPs should be rated to perform at pressures in excess of any anticipated well pressures or kick control injection pressures being approximately 10 to 15 kpsi (69–103 MPa). A minimum of one annular BOP is provided above the ram BOPs to cater for any tubular diameter or for stripping in or out under pressure. An annular BOP is a hydraulically energized elastomeric toroidal unit that closes and seals on varying diameters of tubular member whether stationary or moving into or out of the well. Due to the nature of this pressure barrier element, a lower maximum rated working pressure of about 5 kpsi (34 MPa) is normally available.
Above the annulars, there are no further well pressure barrier elements with the riser only providing a hydrostatic head, liquid containment and guidance of equipment on a normal pressure controlled drilling operation. For a subsea riser system, the hydrostatic head of the different drilling liquids over the ambient sea water pressure means the low pressure zone above the subsea BOP assembly must still withstand, depending upon the depth of water, 5 kpsi (34 Mpa).
The conventional BOP assembly in effect provides a three zone pressure containment safety system. The three zones typically consists of the first high pressure lowermost section encompassing the rams, the medium pressure second zone, the annular or annulars and the low pressure third zone being the bore above to atmosphere and on a subsea system the riser bore to the surface vessel.
It is therefore important to be aware that BOP assemblies need to be tested rigorously in order to verify their full working order and that any potential problems can be identified and rectified before any emergency arises in order to maintain the integrity of a BOP assembly once it is in place. in deep water, BOP assemblies could remain subsea for several months. It is necessary for it to be fully tested at regular intervals and, throughout the subsea industry, this is typically at least once every week.
It is important therefore that the tests on the BOP assembly are carried out carefully and methodically to detect any potential problems but in a reasonable time to minimize risk exposure as testing prevents further downhole well operations especially if the well is open being partially drilled or when involved in a completion or work over. In the case of subsea wellheads which can be at a water depth of as much as 10,000 feet (3050 m), it typically takes approximately three to four hours plus to run the test apparatus into place and three to four hours plus to pull back to the surface after testing has been completed. A typical test sequence takes approximately 6 hours plus to complete if there are no queries or questionable readings. Thus, it is not unusual for a well to be out of operation for approximately 12 hours per week. This is clearly very significant in terms of risk exposure and lost revenue for the well owners and anything which can reduce the well downtime is therefore of great benefit.
Diagnosing any queries or questionable readings can take time even on an integral system, the variety being due to fluid compression, thermal changes of the fluids or to the equipment containing the fluids, riser/vessel movement and the large volumes in the choke and kill lines to the surface in comparison to the relatively small volumes of the BOP cavities and that of a small leak.
A faulty diagnosis or incorrect interpretation due to vague information could lead to the well being temporarily suspended and the BOP assembly being pulled. In deepwater it could take 6 days plus before well operations are resumed.
It is normal procedures when testing the BOP assembly to use a drill pipe or a test mandrel connected above a wellhead tool that will seal within the wellhead. It is also known to try to combine some of the BOP assembly tests with wellhead and surface manifold testing. When testing the BOP assembly it is necessary to ensure that all of the valves, seals, rams and annulars are tested to their maximum expected usage pressure. Each pressure test should be started by a minimum 5 minute low pressure test (e.g. at 300 psi) and then raised in increments to the final high test pressure. Typically, a wellhead/BOP test pressure that is stable and recorded for a minimum of 5 minutes is considered satisfactory. BOP rams are only designed to seal off pressure from below which means all tests have to be carried out either against the wellhead test tool or the well bore. The usual practice is to supply the test pressure to the BOP cavity under test alternating between the choke and kill lines to allow all functions on each side of the BOP stack to be tested from the bore outwards.
When testing the BOP assembly cavities around the test tubular, the BOP test pressures at certain stages of the well could exceed the pressure rating of the well casing so far installed. If a leak occurred from the BOP bore test past the wellhead test tool, the well could be pressured up and be hydraulically fractured, thus making the well unusable. To prevent this occurring the well fluid is allowed to vent up the bore of the wellhead test tool into the bore of the drill pipe where any leak can be monitored on the surface. One particular and critical test is the integrity of the shear blind ram BOP cavity. The shear blind rams are those which can cut the drill string or a pipe or tubing and then seal the BOP bore when there is a need to carry out an emergency disconnect of the riser system from the BOP stack. This, in effect, is the last and only resort for shutting down the well as when the pipe rams are closed on a tubular, the bore of the tubular is still open. Typically, the testing of the shear blind rams requires disconnecting the drill pipe or part of the test mandrel below the shear blind rams and pulling the upper part clear such that the shear blind rams can close.
However, after the mechanical release from the lower part of the test mandrel attached to a wellhead test tool, the bore through the remaining test equipment into the wellhead must be isolated to test up under the shear blind rams. This can be achieved by using either a one way flow mechanism which has the possibility to weep or leak, pressuring up the well casing or alternatively by tripping out of the hole and running a solid wellhead test tool. Either way, after the mechanical release or if a solid wellhead test tool is run, the integrity of the wellhead test tool to seal off in the wellhead cannot be verified before tested.
Even though the shear blind ram BOP cavity is a critical zone to test, the consequences of jeopardizing the integrity of the well casing is deemed too high a risk. Therefore, it is normal practice to test the shear blind ram BOP cavity only to the operationally safe allowable low casing working pressure using either no wellhead test tool or a test tool knowing that, if it leaked, no well damage can occur.
Furthermore, the test liquid pumped and measured on the vessel is supplied at the test pressure typically through either the choke or kill lines down to the appropriate test path into the subsea BOP bore. In addition, this conventional test procedure using the choke and kill lines involves a high volume relative to the small tested cavity volume above the wellhead test tool and in relation to any leaks, meaning that it is difficult to detect leaks.
To reduce premature damage to equipment and function elements, the operation and resetting of the BOP barriers means the valves, rams and annulars should only be opened or closed in a depressurized bore.
Therefore, the choke and kill lines must be vented down between each cavity test, i.e. they are depressurized and repressurized with tests only commencing after the pressure has balanced and stabilized. This is a time consuming process which greatly lengthens the testing time. The compressibility of the drilling liquids, usually drilling mud, and possible expansion or elongation of the lines to the BOP and variations in temperature all contribute to the difficulty of monitoring very small changes in the volume. A wise practice is to circulate the system with seawater which can reduce these effects but not eliminate them entirely.
Once a stable test pressure is achieved, the current BOP testing technique is to surface monitor the test pressure and establish a decay profile. However, when testing, there is a degree of interpretation required as to whether the decays are caused by the above mentioned side effects or a leak. This interpretation has to be carried out by personnel at the surface of the well and is based on experience and judgement rather than facts.
When drilling a well, the prime barrier to prevent an influx of formation fluid is provided by the hydrostatic head of the drilling mud column. It is essential that the consistency and properties of the drilling mud are as specified for certain sections of open hole. This is achieved by circulating a constantly surface trimmed liquid at a designated rate in relation to the liquid properties. In addition, any traces of an influx can be detected by the surface monitoring systems on the return line.
A stationary column of well liquid could unknowingly allow migration of formation fluid into the well bore and the properties of the well liquid could change due to deterioration, thus creating an unstable situation which could result in a kick. Therefore, allowing an open hole to stand stationary for any period of time is an unwise practice. Also, if a kick occurs, the optimum solution is to circulate the kick out under pressure which involves having a tubular member in the hole below the influx and preferably near the bottom of the hole.
Therefore, when having to test a BOP on a well with a balanced open hole, it is a wise practice to use part of the drilling string hung-off below the wellhead test tool. This means that after completing the BOP testing, the well fluids can be circulated and conditioned prior to opening the BOP and pulling the string up to remove the test tools. If a kick has occurred or occurs while pulling out of the riser, the BOP rams can be closed on the drill string and the well circulated. This cannot be achieved if there is a one-way upward flow mechanism in the wellhead test tool or a solid wellhead test plug has been used which would prevent circulation, endangering the operation.
U.S. Pat. No. 4,554,976 discloses a means of testing the shear blind rams of a BOP by splitting the tool into upper and lower portions. In order to test the rams, the upper portion of the tool is removed, the rams tested, and the tool reconnected before withdrawing the tool from the BOP.
U.S. Pat. No. 6,032,736 (Nutec) discloses a test mandrel for use in subsea testing of BOPs which allows the BOP test fluid to be pumped down the drill pipe to a telescopic arrangement. However, this has inherent problems due to possible leakage between the seals of the telescopic portions which makes it very difficult to distinguish a failed BOP. Accounting for the different heights of the wellhead test plug at different stages of the well is accounted for by using spacer pipes between the wellhead test plug and the telescopic test tool. Circulation of the well after testing is not possible unless wireline is run down the drill pipe to remove the blanking dart.
Also monitoring for leaks from around the wellhead test tool is via the test assembly into the drilling riser which has an immense volume in deep water. A means of testing the shear blind rams is not discussed.
SUT Paper (Society of Underwater Technology, UK)—“Acoustic BOP Test Tool” provides additional screwed sections of pipe which can be added to the drill pipe or test mandrel such that the tubular section can be set at the right height in the BOP stack for the different drilling phases.
This would also cater for the use of different wellhead test tools and to land in the wellhead at the different landing shoulders provided by the different casing hangers/seal assemblies as the well is drilled. The height of the tubular test assembly can be changed to meet the BOP space out. An acoustic pressure emitter can be included in the lower part of the test mandrel which transmits the pressure readings up the drill pipe to the surface. A mechanical communication path is required between the emitter and the surface. Again, circulation of the well and testing of the shear blind rams has not been discussed.
This description has mainly addressed the testing of BOP assemblies as multi-closure safety devices as a barrier in the drilling mode. Similar criteria applies when the BOP assembly is used when installing a completion in combination with a completion riser which means the BOP assembly is a critical high pressure isolation mechanism.